The present invention relates generally to pressure relief valves, and more particularly to a pressure relief valve for venting expanding fluids contained in large closed containers. More particularly the present invention relates to a pressure relief valve for venting expanding fluids contained in trucks and rail way tank cars. Railroad tank cars that transport fluids generally comprise two categories: low pressure (general purpose) tanks and pressure tanks. The present invention most particularly addresses pressure tanks which require pressure relief valves. The primary purpose of the pressure relief valve is to vent fluids within the tank and thereby prevent or delay tank failure from increasing internal pressure. In addition, the pressure relief valve should alert persons to evacuate rapidly in the immediate vicinity of a dangerous pressure increase.
A pressure relief valve for a railroad tank car can be internal or external. With internal pressure relief valves the majority of valve components, including springs, are positioned within the tank. Therefore, these components are continuously exposed to the fluid being transported. External pressure relief valves are positioned upon on the exterior tank surface and consequently fewer components are exposed to the transported fluid. The current invention exclusively comprises external pressure relief valves. Unfortunately, in the past high flow rates in the range of 35,000 through 45,000 standard cubic feet per minute were not achieved with external pressure relief valves. The problem to overcome was the fact that traditional pressure relief valves comprise helical compression springs that are prohibitively large for high valve flow rates. In particular, government regulations require that valves comprise a protective housing with a maximum height of approximately thirteen inches. However, a comparable helical spring valve with the same potential spring force as the current invention must be at least seventeen inches in height.
Certain liquids and gases transported in railway tank cars or tank trucks are particularly hazardous and at elevated temperatures. These gases and liquids may expand within the tank and increase the internal pressure within the closed container or tank to a dangerous level. Consequently, government regulatory agencies require operators of these vehicles to install safety pressure relief valves. These pressure relief valves are initially calibrated to automatically open above a pre-set pressure level and thereafter vent the liquid or gas at a specified discharge rate.
Current existing pressure relief valves are generally biased to a closed pre-vent position, such as by a coil spring or a constant-force spring. Helical coil spring valves generally require the internal pressure within the tank to continue to increase beyond the initial valve opening pressure as a condition to achieving the pre-set maximum valve venting capacity. On the other hand, constant-force spring valves have the advantage of (i) opening to a maximum venting capacity instantly at the pre-set opening pressure and (ii) constant-force spring valves do not require the internal pressure within the tank to increase to achieve the pre-se opening pressure.
To overcome this problem the current invention utilizes vertically aligned constant force spring assemblies made of numerous leaf springs. The stacked configuration is an improvement that results in a valve with smaller dimensions. Stacking also allows a substantially increased spring force compared with existing pressure relief valves. The spring assemblies attach to a singe spring block with spring bolts and results in an increase flow of fluid within this smaller confined area.
Unfortunately, size, complexity and cost of the constant-force spring valve required for this particular closed tank application have prevented their general acceptance in the industry. Space restrictions along the tank or other closed container, as well as design constraints originally limited the number of constant force springs to offset this space and design restriction. U.S. Pat. No. 5,855,225 (Williams III) solved this problem with four constant force springs. In Williams, each constant force spring comprised rolled leaf springs positioned along the same horizontal plane within the valve.
To further overcome the above problems, in the current invention spring assemblies containing leaf springs are now (i) vertically aligned along two horizontal planes and (ii) attached to a structure known as the spring block. This vertical alignment of the spring assemblies attach to a spring block by spring bolts, and this alignment also improve the performance of the entire valve.
The current invention also preferably includes a bearing assembly positioned between the adjustment screw and the sealing disc, and therefore less torque to required rotating the adjustment screw. The bearing assembly also reduces the friction between the adjustment screw and disc, and thereby eliminates the requirement for constant lubrication of the contact area between these two components. The spring bracket also provides a more rigid guide for vertical movement of the spring block during valve operation. The spring bracket also attaches to a spring bracket brace and thereby further improves the performance of the pressure relief valve.
The current invention also provides a roller bearing assembly to reduce friction and wear between rotating and contacting valve components. For example, U.S. Pat. No. 5,855,225 (Williams) discloses two contacting surfaces between the adjustment screw and seal disc that rely upon lubrication to prevent friction. However, with the increased spring assembly force capacity of the current invention, corresponding increased wear and friction would (i) damage these surfaces and (ii) result in a prohibitively high torque requirement to rotate the valve's adjustment screw. Fortunately, this new roller bearing assembly prevents this wear, friction and constant lubrication task for higher valve forces.